1. Technical Field
The present invention relates to telephone instruments and more particularly, to an amplifier for use in a telephone instrument which monitors the status of a telephone call while leaving the handset onhook.
2. Background Art
Since the advent of the electronic telephone many different features have been added to the telephone instrument. One such feature is the ability to monitor the progress or status of a call with an amplified signal to a loudspeaking device or similar transducer in order for the user to be able to hear signals. In this manner conditions such as dial tone, busy signal, ring-back signal and voice answer can be heard from a speaker or similar transducer inside the telephone instrument. In this mode the hookswitch is connected by a relay or other parallel switching device to maintain the telephone instrument in the operated condition.
This monitor feature is similar to that found in the so called loudspeaking telephones wherein the voices from the receive channel are amplified and applied to the speaker so that the received voice can be heard without holding a handset to the user's ear. Such loudspeaking telephones usually require an external source of power other than the telephone line itself. This is particularly true since it is difficult to drive a loudspeaker from the telephone line to sufficient loudness without draining excessive current from the telephone instrument itself or from the telephone line. This is particularly important because the telephone instrument must function properly so that such functions as loop supervision and address signalling can take place while in the monitor mode.
Most prior art circuitry of this type has utilized operational amplifier circuits to provide the monitor function. Such circuits generally require sources of both positive and negative voltage i.e. +15 volts and -15 volts DC as well as requiring the utilization of a significant number of capacitors and resistors to support the integrated circuit operational amplifiers. Some of the circuits make use of integrated audio amplifiers. Such amplifiers are expensive and sometimes critical in their application within the telephone instrument. An example of this is found in the use of the LM384 integrated circuit manufactured by National Semiconductor Inc. which is critical as to the layout of circuit traces and component locations on the printed circuit board. The GTE 2000 series of telephones manufactured by GTE Communication Systems Corporation is an example of the use of commercially available integrated circuit amplifiers for the monitor function. In such applications the type of amplifier used must be capable of high gain and low distortion preferably with a low impedance miniature loudspeaker.
Many early prior art designs required relatively high values for loudspeaker impedance in many instances being greater than 40 Ohms. A much more desirable range of loudspeaker impedance is in a range from 4 to 16 Ohms because of the greater economy and availability of such units. It should be noted that there is also a very limited variety of available integrated circuit audio amplifiers suitable for telephone line powered use.